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Sedimentology and Petroleum Occurrence, Schoolhouse Tongue of Weber Sandstone (Lower Permian), Northwest Colorado

Samuel Y. Johnson, Christopher J. Schenk, Donald E. Anders, Michele L. Tuttle

The Schoolhouse Tongue of the Weber Sandstone, an extensive paleo-petroleum reservoir in northwest Colorado, consists mainly of bleached or oil-stained sandstone of inferred eolian sand-sheet origin. Low-Previous HitangleNext Hit to parallel-bedded, very fine to fine-grained sandstone is the dominant facies. Low-Previous HitangleNext Hit deflationary surfaces and deflation lags are Previous HitcommonNext Hit. Cross-bedded dune deposits are a less Previous HitcommonTop sand-sheet facies. Interbedded fluvial deposits are present in most sections. The sand-sheet deposits of the Schoolhouse Tongue are sedimentologically similar to those in the gradationally underlying red beds of the Middle Pennsylvanian to Lower Permian Maroon Formation, and the Schoolhouse Tongue is best construed as the uppermost sand sheet in the Maroon sequence.

At Rifle Creek, the site of a late Paleozoic-early Mesozoic structural high, the Schoolhouse Tongue is 66 m thick and oil staining extends several hundred meters down into the underlying Maroon Formation. Away from Rifle Creek, the Schoolhouse Tongue thins to the north and pinches out to the southeast and east (within 40-65 km), and oil staining in the Maroon is minimal. The distribution of oil-stained rock suggests that hydrocarbons were introduced at a point source, possibly related to faults on the margins of the paleohigh. Oil in the Schoolhouse Tongue mainly occurs in secondary pore space resulting from the dissolution of carbonate cement by migrating organic acids. Oil was trapped below overlying red siltstones. Geochemical typing of the hydrocarbons is consistent with a late Pa eozoic source rock.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.